1. Field of the Invention
The present invention relates to a rod-shaped light guide for use in a facsimile machine, a copying machine, a scanner, etc., and an illuminating device which incorporates such a rod-shaped light guide.
2. Description of the Related Art
Facsimile machines, copying machines, scanners, etc. employ an image reading device such as an image sensor for reading subjects. One type of such an image reading device comprises a contact-type image sensor having a short light path length which allows the image sensor to be easily incorporated in those machines and scanners.
The contact-type image sensor reads a section of the subject which is being illuminated by an illuminating device to an illuminance level that is required to read the section of the subject. The section of the subject which is illuminated in a strip-like range that is considerably long in a main scanning direction, i.e., a longitudinal direction thereof, and short in an auxiliary scanning direction perpendicular to the main scanning direction.
In order to illuminate such a narrow strip-like range, the illuminating device heretofore comprises an array of LEDs (light-emitting diodes) mounted on a printed-wiring board by wire bonding or soldering. The array of LEDs tends to produce a waste of illuminating light because effective light that is required is only light applied to the section of the subject which needs to be read. In addition, it is costly, laborious, and time-consuming to install a number of LEDs on the printed-wiring board.
Japanese laid-open patent publications Nos. 8-163320 and 10-126581 disclose an image sensor having a rod-shaped light guide.
As shown in FIG. 7 of the accompanying drawings, the disclosed image sensor comprises a rod-shaped light guide 101 mounted in a casing 100. The rod-shaped light guide 101 has a basically rectangular cross-sectional shape lying in a direction perpendicularly to the longitudinal direction thereof which is normal to the sheet of FIG. 7. The rod-shaped light guide 101 has a beveled corner surface 102 serving as an exit surface. The image sensor also has a light-emitting element (not shown) such as an LED mounted on an longitudinal end of the rod-shaped light guide 101. Light emitted from the light-emitting element is introduced as illuminating light into the rod-shaped light guide 101 from its longitudinal end. The illuminating light as it is propagated through the rod-shaped light guide 101 is scattered by a light scattering pattern 103 formed on a side of the rod-shaped light guide 101. The scattered light is emitted from the exit surface 102 and transmitted through a cover glass panel 104 as a subject holder to the image surface of a subject placed on the cover glass panel 104. Light reflected from the image surface is focused by a rod lens array 105 onto a photoelectric transducer 106, which reads the image of the subject.
In the above conventional arrangement, the light scattering pattern 103 is shaped in order to reduce light intensity irregularities in the longitudinal direction of the rod-shaped light guide 101, i.e., the main scanning direction. However, a spatial light intensity distribution in the auxiliary scanning direction remains to be improved.
Specifically, illuminating light emitted from the exit surface of a rod-shaped light guide which has a rectangular or similar cross-sectional shape contains light reflected in multiple paths or scattered in the light guide in addition to scattered light produced by the light scattering pattern, as shown in FIG. 8 of the accompanying drawings. Therefore, such illuminating light fails to achieve an optimum illuminated state on the image surface to be read in the auxiliary scanning direction.
More specifically, the conventional rod-shaped light guide, which does not have a condensing function and a function to adjust the peak of a spatial light intensity distribution is liable to produce multi-reflected light and scattered light that are more intensive than the scattered light from the light scattering pattern. Thus, the peak of the spatial light intensity distribution in the auxiliary scanning direction is shifted toward the rod-shaped light guide, i.e., to the right in FIG. 8, out of a position directly above the rod lens array.
The above drawback can be reduced to a certain extent by shortening the distance between the rod-shaped light guide 101 and the rod lens array 105. However, if the distance between the rod-shaped light guide and the rod lens array is shortened, the illuminating light from the rod-shaped light guide may be reflected directly by a surface of the cover glass panel 104, or reflected by the surface of the subject, if the subject is lustrous, resulting in bright reflections from the image to be read.
Another solution is to tilt the rod-shaped light guide 101 to move the peak of the spatial light intensity distribution away, i.e., to the right in FIG. 8, into alignment with the position directly above the rod lens array. This approach, however, is not effective because it increases the extent of the spatial light intensity distribution.
It has been known to make the exit surface convex to converge the emitted illuminating light. With this proposed, however, it is difficult to make adjustments to position the peak of the spatial light intensity distribution in alignment with the position directly above the rod lens array, and the space availability in the image sensor is limited by the convex exit surface.
It is therefore an object of the present invention to provide a rod-shaped light guide which is capable of bringing the peak of a spatial light intensity distribution in an auxiliary scanning direction into alignment with an image surface to be read.
Another object of the present invention is to provide an illuminating device which incorporates such a rod-shaped light guide.
According to the present invention, there is provided a rod-shaped light guide for reflecting illuminating light entered from one end thereof with inner surfaces thereof and emitting reflected illuminating light from an exit surface lying in a longitudinal direction thereof, the rod-shaped light guide having side surfaces extending in the longitudinal direction thereof, the side surfaces including a planar exit surface, a planar surface with a light scattering pattern disposed thereon, and a curved reflecting surface for reflecting scattered light from the light scattering pattern toward the exit surface, the curved reflecting surface having a cross-sectional shape which lies perpendicularly to the longitudinal direction and comprises a portion of an elliptical shape, the light scattering pattern being disposed near a focus of the elliptical shape.
With the above arrangement, the light scattering pattern and an image surface illuminated by scattered light from the light scattering pattern are brought into focused relationship to each other i.e., the scattered light from the light scattering pattern can be focused onto the image surface to be read.
Illuminating light may be introduced into the rod-shaped light guide from opposite ends thereof. The light scattering pattern may be of any desired shape.
The planar surface with the light scattering pattern disposed thereon may include a major axis of the elliptical shape, and the exit surface may include a minor axis of the elliptical shape.
The planar surface with the light scattering pattern disposed thereon may include a major axis of the elliptical shape, and the exit surface may include a side inclined to a minor axis of the elliptical shape.
The planar surface with the light scattering pattern disposed thereon may include a side parallel to a minor axis of the elliptical shape, and the exit surface may include a minor axis of the elliptical shape.
The planar surface with the light scattering pattern disposed thereon and the exit surface may comprise a single surface.
According to the present invention, there is also provided an illuminating device comprising a casing, a rod-shaped light guide for reflecting illuminating light entered from one end thereof with inner surfaces thereof and emitting reflected illuminating light from an exit surface lying in a longitudinal direction thereof, the rod-shaped light guide having side surfaces extending in the longitudinal direction thereof, the side surfaces including a planar exit surface, a planar surface with a light scattering pattern disposed thereon, and a curved reflecting surface for reflecting scattered light from the light scattering pattern toward the exit surface, the curved reflecting surface having a cross-sectional shape which lies perpendicularly to the longitudinal direction and comprises a portion of an elliptical shape, the light scattering pattern being disposed near a focus of the elliptical shape, the rod-shaped light guide being housed in the casing with the exit surface exposed, and light-emitting means for emitting light, the light-emitting means being mounted on an end of the rod-shaped light guide, the light scattering pattern being progressively wider from the end the rod-shaped light guide toward another end thereof or the light scattering pattern comprising a plurality of discrete elements at least near the end of the rod-shaped light guide.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.